Untightening
of Nuts & Bolts
If
a nut is untightened immediately after being tightened, the
torque needed to untighten it will be less than that needed
to tighten it. This is due to the portion of the torque which
actually stretches the bolt always acting in the untightening
direction. Hence when a nut is immediately untightened, usually
about 20% less torque is needed than was needed to tighten
it in the first place.
As the time passes from when the nut
was tightened, the torque needed to untighten it tends to
increase. After half a day or so, the release torque can be
typically up 10% greater than the tightening torque. This
is due to a number of effects including embedding of the contact
surfaces and changes in temperature affecting the friction
conditions. Such effects typically increase the friction and
hence a greater release torque is required. Because of such
friction changes, the use of torque auditing methods (measuring
the backoff - or crack-off torque or the tightening or crack-on
torque) to assess if the bolt being tightened correctly is
only accurate if it is completed shortly after the assembly
was tightened.
>The greater the changes in the temperature
and environment experienced by a bolted joint, the more rapid
are the changes in the friction and subsequent change to the
release torque. In subsea applications, in applications sustaining
elevated or low temperatures, significant changes in the friction
conditions can rapidly occur. The properties of any lubricant
will change over time, which in some applications, can lead
to galling of the surfaces resulting in a dramatic increase
in the release torque i.e. by as much as 50% to 100% of the
tightening torque.
Over a prolonged time period, oxidation
and corrosion of the thread interfaces and nut face can occur
making removal of the nuts problematic without destroying
the bolt. In such circumstances nut splitters and similar
measures are needed for nut removal.
When hydraulic torque tightening tools are used, problems can occur
in that the tool used to tighten the nut cannot remove it.
Hence it is normal practice for the tool to be selected based
upon 80% of its rated torque value to allow some margin for
subsequent nut removal. Even with such an allowance, a larger
rated tool may be needed to remove the nut under certain circumstances.
To assist in preventing problems, avoid
having too much thread protrusion past the top of the nut
(more than two thread pitches unless hydraulic tensioners
are used). Also consider the use of protective caps that pass
over the thread/nut to protect the surfaces from the effects
of corrosion and incidental damage.
|